Existing epoxy products for coating a ballast-tank of a partly or fully submersible structure, such as a vessel or a semi submersible oil rig, show relatively high crack susceptibility when applied in extreme high dry film thickness. Cracks may occur during launch or during service in places with too high dry film thickness. Too high dry film thickness is especially observed in association with the interior of the angles formed between the structural elements, where the shape of the surface is such that a paint film with even thickness is seldom achieved. Existing epoxy products are typically specified in a total dry film thickness of 250-500 μm, but in association with the interior of the angles formed between the structural elements there is a risk of getting far higher dry film thicknesses than specified. Too high coating thickness, often exceeding normal acceptance limits, causes high curing shrinkage stress and may enhance cracking.
The factors leading to cracking of ballast tank coatings are studied in recent articles by Askheim et al. (“Why do paints crack. Ballast tank coating study focusing on brittleness and loss of flexibility” Protective Coating Europe, March 2001, pp 49-55) and Lim et al. (“Stress Analysis and Evaluation of Cracks Developed on the Coatings for Welded Joints of Water Ballast Tanks” Paper presented at “Corrosion 2005”, Houston, USA).
The epoxy coat in a ballast tank is exposed to many stress inducing environmental effects, E.g. special environmental effects in ballast tanks of vessels include elevated temperature on areas, such as bulkheads adjacent to cargo holds and engine rooms, cyclic sun-heating underneath the main deck, water sloshing, reverse impacts from heavy loading equipment, and hydrostatic pressure changes from ballasting/de-ballasting cycles. The environmental effects give special demands to the flexibility and cracking resistance of an epoxy coat applied in the ballast tank of a partly or fully submersible structure. Cracks in the paint film will lead to corrosion due to the presence of water, typically sea water, when ballast is required.
The brittleness of the epoxy coat is believed to be related to limitations in the flexibility of basic epoxy chemistry. The epoxide group can react with various curing agents. Epoxide groups can also homopolymerise with themselves to varying degrees. The end result is a three-dimensional network of long chain molecules with limited flexibility. The flexibility varies with the type and amount of curing agent as well as type and amount of flexibilisers. If much more than the stoichiometric amount of curing agent is added to the epoxy resin, the flexibility is improved, but mechanical strength and anticorrosive properties are reduced.
JP 59-78267A discloses a coating composition comprised of an epoxy resin, a curing agent, an inorganic staple fiber and a coating compounding agent. The coating composition is mainly used for a corrosion-proofing coating at the inside and outside of steel pipes.
A method is required for the establishment of a crack resistant epoxy paint coat at the surface represented by the interior of the angles formed between structural elements of a ballast tank of a partly or fully submersible structure.